1. Field of the Invention
The present invention relates to an optical fiber applicable to an optical fiber amplifier and a process of producing the optical fiber.
2. Description of the Related Art
An optical amplifier capable of directly amplifying light signals without conversion into electric signals is advantageous in that it can be easily enlarged in capacity because of a substantially bit rate free function thereof and that it can collectively amplify multiple channels, and from this standpoint, it is being extensively studied as one of key devices of future optical communication systems by various research organizations.
There is known one form of an optical amplifier using a single mode optical fiber including a core doped with a rare earth element such as Er, Nd, or Yb (hereinafter, referred to as xe2x80x9ca doped fiberxe2x80x9d), wherein signal light to be amplified is transmitted to the doped fiber and at the same time pumping light is introduced into the doped fiber in the direction identical or reversed to that of the signal light.
The optical amplifier using the doped fiber, which is called an optical fiber amplifier, has excellent features of eliminating a polarization dependency of a gain, lowering a noise, reducing a loss in coupling with an optical transmission path, and the like. In practical use of the optical fiber amplifier of this type, it is required to make wider a wavelength band width of signal light in which the signal light can be amplified at a specific gain (hereinafter, referred to simply as xe2x80x9ca wavelength band widthxe2x80x9d) and to make higher a conversion efficiency of pumping light into signal light.
As for light having a wavelength within a range of 0.8 to 1.6 xcexcm, there have been established a technique of producing an optical fiber using a quartz glass suitable for long-distance transmission, and a technique of putting the optical fiber into practice. An optical fiber is obtained by drawing a preform in the shape of a thick rod. The preform is required to have a composition gradient in a cross-sectional direction thereof which is accurately set as designed.
A standard process of preparing a preform has been known, in which a glass composition chemically converted from reactive gases is deposited on an inner surface of a quartz reaction tube by a MCVD (Metal Chemical Vapor Deposition) process or the like. In the MCVD process, suitable reactive gases such as SiCl4 and O2 are introduced in the quartz reaction tube, and the quartz reaction tube is heated at a temperature suitable for reaction of the gases. A heating zone is moved in the longitudinal direction of the quartz reaction tube, to deposit a new glass layer on an inner wall surface of the quartz reaction tube. A plurality of layers, for example, 20-30 layers are repeatedly deposited. A composition gradient in the cross-sectional direction of an optical fiber produced from the preform can be controlled by independently adjusting compositions of the layers of the preform. After the layers are fully deposited, the quartz reaction tube is collapsed by heating, to be thus formed into a rod-shaped preform. The preform is then drawn to produce an optical fiber.
In the MCVD process, a reactive material vaporized at a room temperature is generally used. For example, SiCl4 is used for forming SiO2 which is a main component of an optical fiber, and GeCl4 is used for forming GeO2 which is an element for adjustment of a refractive index. Incidentally, for production of a doped fiber, a suitable reactive material containing a rare earth element sufficiently evaporated at a room temperature cannot be obtained, differently from SiCl4 and GeCl4, and consequently a rare earth element cannot be doped in the doped fiber at a practically sufficient concentration only by the MCVD process. For this reason, a rare earth element has been doped in a doped fiber at a practically sufficient concentration in the following manner.
A known process of preparing a preform suitable for production of a doped fiber includes a step (1) of depositing a soot-like core glass on an inner surface of a quartz reaction tube, a step (2) of allowing the soot-like core glass impregnated with a solution containing a rare earth element compound as a solute, and a step (3) of drying the solution and collapsing the quartz reaction tube. On the other hand, a technique of widening a wavelength band width of an optical fiber amplifier using a doped fiber has been proposed, in which a core is impregnated with Al2O3 as well as a rare earth element.
For example, Japanese Patent Laid-open No. Hei 5-119222 discloses a double core structure including an aluminum/silica based glass (Er-Al-SiO2) doped with erbium (Er) and aluminum (Al), which is provided at a center portion of a core; and a germanium/silica based glass (Ge-SiO2) doped with germanium (Ge), which is provided at an outer peripheral portion of the core. In the prior art structure disclosed in Japanese Patent Laid-open No. Hei 5-119222, however, is disadvantageous in that a relative index difference Al of the core peripheral portion is about 2% but a relative index difference xcex942 of the core center portion is about 0.7% at the utmost, with a result that there occurs a large depression of a refractive index at the core central portion.
This is due to the fact that an element doped for widening a band width, such as Al, acts to decrease a refractive index. The depression of a refractive index causes a phenomenon in which a mode field of transmission light is spread and thereby a mode field diameter is made larger. The mode field diameter thus increased is inconvenient in converting pumping light into signal light and results in degradation of a conversion efficiency of pumping light into signal light. For example, in the prior art structure disclosed in Japanese Patent Laid-open No. Hei 5-119222, a mode field diameter was about 4.8 xcexcm and a conversion efficiency of pumping light into signal light was 64%.
It is therefore an object of the present invention to provide an optical amplifying fiber capable of widening a wavelength band width, and improving a conversion efficiency of pumping light into signal light by suppressing a mode field diameter at a small value, and to provide a process of producing the optical amplifying fiber.
In accordance with an aspect of the present invention, there is provided an optical amplifying fiber including: a clad having a first refractive index; a first core, provided inside the clad, containing a refractive index increasing element and having a second refractive index higher than the first refractive index; a second core, provided inside the first core, containing a rare earth element and an amplification band width widening element and having a third refractive index higher than the first refractive index and lower than the second refractive index; and a third core, provided inside the second core, containing a refractive index increasing element and having a fourth refractive index higher than the third refractive index.
The refractive index increasing element may be selected from Ge and Ti. Of these elements, Ge is preferably used. The second core preferably contains Er and Al.
According to another aspect of the present invention, there is provided a process of producing an optical amplifying fiber, including the steps of: (a) forming a first core layer mainly made of SiO2 doped with GeO2 or TiO2 on an inner surface of a quartz reaction tube by chemical vapor deposition; (b) forming a soot-like second core layer mainly made of SiO2 on the first core layer by chemical vapor deposition; (c) allowing the second core layer impregnated with a solution containing a rare earth element and at least one element selected from a group consisting of Al, Zn, Sn and La; (d) evaporating a solvent of the solution impregnated in the second core layer; (e) heating the second core layer to vitrify the second core layer; (f) forming a third core layer mainly made of SiO2 doped with GeO2 or TiO2 on the second core layer by chemical vapor deposition; (g) perfectly collapsing the quartz reaction tube by heating to form a preform; and (h) melting and spinning the preform.
In the optical amplifying fiber of the present invention, since the third core having a large relative index difference is provided at the core central portion, a light power is concentrated at the core central portion as compared with the doped fiber having the prior art structure, to make smaller a mode field diameter, thereby improving a conversion efficiency of pumping light into signal light.
Further, since the second core contains a rare earth element and an amplification band width widening element, it is possible to ensure a sufficient wide band width of an optical fiber amplifier. The amplification band width widening element may be selected from a group consisting of Al, Zn, Sn, and La.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.